::quote-start and ::quote-end, to match characters with the Quotation_Mark property that are direct children of the element and that are immediately inside the start and end (respectively) of the element, ignoring White_Space characters. (This would help a lot with <q> in HTML5.)

:placeholder - matches form elements displaying placeholder text. This allows web developers and theme designers to customize the appearance of placeholder text, which is a light grey color by default. This may not work well if you've changed the background color of your form fields to be a similar color, for example, so you can use this pseudo-class to change the placeholder text color.